下面代码段实现了快速排序的划分操作（以首元素为基准），横线处代码应填入
def partition(arr, low, high):
pivot = arr[low]
i = low
j = high
while i < j:
while i < j and arr[j] >= pivot:
j -= 1
while i < j and arr[i] <= pivot:
i += 1
if i < j:
arr[i], arr[j] = arr[j], arr[i]
________________ # 在此处填入代码
return i
arr[low], arr[high] = arr[high], arr[low]
arr[low], arr[i] = arr[i], arr[low]
arr[i], arr[high] = arr[high], arr[i]
arr[i] = pivot